18 Questions
What is the purpose of the 'adapter' in the link layer?
To implement the link layer protocol
What is the function of the sending side in the link layer?
To encapsulate datagrams in a frame and add error checking bits
What is the goal of the Internet checksum?
To detect errors in the transport layer
What is the role of the receiving side in the link layer?
To extract datagrams and pass them to the receiving node
What is the purpose of error detection and correction bits (EDC)?
To detect and correct errors
What is the limitation of error detection?
It may miss some errors, but rarely
What is the primary purpose of a LAN address?
To get a datagram from one interface to another physically-connected interface
How are MAC addresses allocated?
By the IEEE, which administers the MAC address space
What is the size of a MAC address in most LANs?
48 bits
What is the analogy for a MAC address?
Like a Social Security Number
What is the purpose of ARP?
To translate IP addresses to MAC addresses
What is the size of an IP address?
32 bits
What is the primary function of the ARP protocol?
To resolve IP addresses to MAC addresses
What is the purpose of the TTL in an ARP table?
To specify the time after which an ARP entry will be forgotten
How does a device on a LAN typically obtain the MAC address of another device on the same LAN?
By broadcasting an ARP query packet
What is the main purpose of the preamble in an Ethernet frame?
To synchronize the sender and receiver clock rates
What happens when a device on a LAN receives an Ethernet frame with a destination address that does not match its own MAC address?
The device discards the frame
What is the term for information that times out or goes away unless it is refreshed?
Soft state
Study Notes
Link Layer and ARP
- A wants to send a datagram to B, knowing B's IP address, but needs B's MAC address
- ARP (Address Resolution Protocol) is used to determine MAC address of B
- Each IP node (Host, Router) on LAN has an ARP table, which maps IP addresses to MAC addresses
- ARP table entries have a Time To Live (TTL) of typically 20 minutes
ARP Protocol
- A sends an ARP query packet with B's IP address, which is broadcast to all machines on the LAN
- B receives the ARP packet and replies to A with its MAC address
- A caches the IP-to-MAC address pair in its ARP table until the information times out
- ARP is "plug-and-play" and nodes create their ARP tables without intervention from the network administrator
Ethernet Frame Structure
- Preamble: 7 bytes with a pattern of 10101010 followed by one byte with a pattern of 10101011
- Addresses: 6 bytes (destination MAC address)
- Type: indicates the higher layer protocol (mostly IP, but others may be supported)
- CRC: checked at the receiver, and if an error is detected, the frame is dropped
Adapters and Link Layer Protocol
- Link layer is implemented in the "adapter" (aka NIC or Ethernet card)
- Sending side: encapsulates the datagram in a frame, adds error checking bits, and more
- Receiving side: looks for errors, extracts the datagram, and passes it to the receiving node
Error Detection and Correction
- Error Detection and Correction (EDC) bits are added to the data for error detection
- Error detection is not 100% reliable, and larger EDC fields yield better detection and correction
- Internet checksum: detects "errors" (e.g., flipped bits) in transmitted segments at the transport layer
- Sender: computes checksum, and receiver verifies it to detect errors
LAN Technologies and Addresses
- LAN technologies: Ethernet, hubs, bridges, switches, 802.11, PPP, ATM
- LAN (or MAC or physical or Ethernet) address: used to get a datagram from one interface to another physically-connected interface
- 48-bit MAC address is burned in the adapter ROM, and allocation is administered by IEEE
This quiz covers the basics of the link layer protocol, including error correction, half-duplex and full-duplex communication, and the role of adapters and frames in data transmission.
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